would be limited to that ice present in the river
to assess the relative effectiveness of construct-
between Fort Fairfield and the dam. The HEC2
ing some type of ICS.
program with ice option was run for this case, for
For this study, we assumed in the computa-
the same discharge of 20,000 ft3/s and with the
tions that all the ice upstream from any control
additional assumption, supported by the field
structure would be held back, thus limiting both
measurements and the calculations in Table 2,
the supply of ice reaching an ice jam in the Fort
that the ice thickness averaged 28 in. The results
Fairfield area and the resulting water levels. The
of the computations are presented in Figure 13.
type and characteristics of the ICSs were not
With ice supply limited by the dam, we found
specified because the effects of the ICS itself on
that the jam should extend up to station 680, ap-
the water surface profile would be relatively
proximately, and that the jam backwater should
small as compared to the ice effects, and they,
merge with the open water surface profile at
also, would be local and would require a more
about station 950. Therefore, improvements to
accurate HEC2 deck than the extended one
the dam to ensure that it acts as an ice-control
used here. All computations were made for the
same flow discharge of 20,000 ft3/s.
structure would not be sufficient to alleviate ice
jam flooding at Fort Fairfield. Additional mitiga-
Figure 14a shows the effect of constructing an
tion means, upstream of Fort Fairfield and below
ICS at station 423+00, approximately 2000 ft up-
the dam, would be required.
stream from the Limestone Road bridge for the
case of unlimited ice supply. Figure 14b shows
the effect of the same ICS when the ice supply is
Effects of ice-control structures upstream
limited by the Caribou dam and for an ice thick-
of Fort Fairfield
As mentioned previously, the primary objec-
ness of 28 in. Both figures show that the water
tive of extending the HEC2 data deck beyond
level is reduced by as much as 6 ft between
the original, verified deck provided by NED was
about stations 330 and 423. Upstream of the ICS
400
ICS
380
No ICS
Open
360
Bed
a. Unlimited ice supply.
340
400
380
ICS
No ICS
Open
360
Bed
b. Ice supply limited by the
Caribou dam (hi = 28 in.).
340
Figure 14. Effects of an ICS at
1200 x 10 2
200
400
600
800
1000
station 423 (Q = 20,000 ft3/s).
Distance (ft)
19
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